When a 1991 cholera outbreak that killed thousands in Peru was traced to plankton blooms fueled by warmer-than-usual coastal waters, linking disease outbreaks to epidemics was a new idea. Now, scientists say, it is a near-certainty that global warming will drive significant increases in waterborne diseases around the world.

Rainfalls will be heavier, triggering sewage overflows, contaminating drinking water and endangering beachgoers. Higher lake and ocean temperatures will cause bacteria, parasites and algal blooms to flourish. Warmer weather and heavier rains also will mean more mosquitoes, which can carry the West Nile virus, malaria and dengue fever. Fresh produce and shellfish are more likely to become contaminated.

Heavier rainfalls are one of the most agreed-upon effects of climate change. The frequency of intense rainfalls has increased notably in the Midwest, the Northeast and Alaska, and the trend will accelerate, said the 2007 report of the United Nations' Intergovernmental Panel on Climate Change.

The consequences will be particularly severe in the 950 U.S. cities and towns -- including New York, the District, Milwaukee and Philadelphia -- that have "combined sewer systems," archaic designs that carry storm water and sewage in the same pipes. During heavy rains, the systems often cannot handle the volume, and raw sewage spills into lakes or waterways, including drinking-water supplies.

On Sept. 13, during an unrelenting downpour, Chicago chose to prevent urban flooding by opening and releasing runoff containing raw sewage into Lake Michigan. About a month later, a University of Wisconsin study published in the American Journal of Preventive Medicine predicted an increase of 50 to 120 percent in such releases into the lake by the end of the century.

"One of the strongest indicators from climate models is more intense rains," said co-author Stephen Vavrus, director of the university's Center for Climatic Research. "They don't agree on everything, but they do agree on that. A warmer atmosphere holds more moisture, so as we get more moisture in the air, when we do have a storm situation, you get more total rainfall."

From 1948 to 1994, heavy rainfall was correlated with more than half of the nation's outbreaks of waterborne illness, according to a 1991 study commissioned by the Environmental Protection Agency. In one of the worst, torrential rains in Milwaukee in 1993 triggered a sewage release that exposed 403,000 people to cryptosporidium, a protozoan parasite transmitted in fecal matter. Fifty-four people died.

"Raw sewage got sucked back into the clean water supplies," said Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. "Cryptosporidium is a parasite that chlorine doesn't kill, so it escaped water treatment."

On Ohio's South Bass Island in Lake Erie in the summer of 2004, at least 1,450 residents and tourists suffered gastrointestinal illnesses linked to several months of above-average rains that contaminated the town's drinking water.

More than 100 pathogens can cause illness if you drink or swim in water contaminated by sewage, including norovirus Norwalk and hepatitis A viruses and bacteria such as E. coli and campylobacter.

"If someone gets something swimming, they could bring it into work or day care. This is what's happened with cryptosporidium before," said Joan Rose, a Michigan State University professor and water researcher. "So we have all these rippling effects that occur in our community."

Combined sewer overflows can be eliminated by upgrading sewerage systems, but it is an expensive process.

"Here we are in a wealthy country with a very strong public health infrastructure," said Jonathan Patz, a professor of environmental studies and population health sciences at the University of Wisconsin at Madison. "But we need to realize it's not as strong as we thought it was, and water systems really need tremendous resources for upkeep in the face of climate change."

A report last week by the National Research Council concluded that the EPA's storm-water program needs major overhauls to deal with increasing runoff, including a more integrated permitting system based on watersheds and a focus on land use by growing municipalities. Benjamin H. Grumbles, EPA assistant administrator for water, said Friday that upgrading combined sewer systems is among the agency's top priorities.

Runoff from agricultural land can also spread waterborne diseases, and rising water temperatures are conducive to the growth of pathogens such as naegleria, an amoeba that enters the nasal passages and leads to often-fatal meningoencephalitis. Warmer waters also trigger blooms of algae and plankton, which themselves can be toxic or can harbor pathogens such as the bacteria that cause cholera, as has happened in Peru and the Bay of Bengal.

Algae blooms are also fostered by nitrogen and phosphorus that are washed into rivers, lakes and the ocean by heavier rainfalls. Downpours are likely to lead to more seafood contamination as human waste, animal manure, nitrogen and phosphorus make their way to coastal areas.

Epstein said the recent flooding in Texas from Hurricane Ike and the mosquito infestation that followed are one example of climatic conditions that are likely to foster more waterborne disease in coming years, despite efforts by the EPA and the Centers for Disease Control and Prevention.

"It will be the next few years. This is not 20 years away," Epstein said. "It's already occurring. The CDC is gearing up to deal with [it], but at the same time, we need to be focused on the primary driver, which is our unstable climate. We need to do all of the above -- protect, prepare and prevent."

(By Kari Lydersen, Washington Post, 20/10/2008)